1. Field of the Invention
The present invention relates to a process for the inscription of moldings based on polyurethane elastomers or mixtures of thermoplastic polyurethane elastomers with up to 45 % by weight, based on the total weight of polymers, of further thermoplastics, by means of high-energy radiation.
2. Description of Related Art
Plastics can be inscribed by printing processes. In recent years, however, laser inscription has become widely established. The advantages are that it is simple, fast and precise to carry out, is non-contact (no mechanical stresses or soiling of the workpiece), flexible (computer-controlled), clean and requires no chemicals. Laser equipment can easily be integrated into automatic production lines; finished plastic parts can be inscribed individually, since a layout change can be made simply and rapidly aided by computer. The inscription is carried out without contact, even in poorly accessible areas. There are thus considerable advantages over conventional printing methods.
The most important characteristic of the inscription produced is legibility, which can be quantified by means of the contrast. In addition to high contrast, an adequate depth of the change in color and a very smooth surface are required. The inscription should be abrasion-resistant and chemicals-resistant.
For a high-quality mark to be formed on a plastic molding, the absorption of laser light must cause a color-change reaction. Depending on the wavelength, incident intensity, pulse length, etc., different interactions between laser light and plastic dominate in each case, and a distinction is therefore made between a number of different laser-induced mechanisms which result in a durable mark: thermochemical reaction, carbonization of the surface, melting, remelting, evaporation, sublimation, engraving, discoloration, foaming, photochemical reaction, ablation (removal of material), bleaching of dyes, removal of a coating film.
In the case of laser inscription, a distinction is made between two processes; mask (projection) inscription and beam-deflection inscription (scanning of the laser beam). In mask inscription, pulsed lasers are used. A laser beam of sufficiently large aperture illuminates a mask carrying all the information to be transferred. By means of a lens, the mask is imaged on the surface to be inscribed; the information can be applied to the workpiece by means of a single laser pulse. In the case of large images, the mask can be scanned with a plurality of pulses. The maximum size of the inscription field is limited by the requisite energy density. The projection process thus allows rapid inscription; however, since a mask must be prepared, it is not so flexible. In the case of beam-deflection inscription, the laser beam is deflected onto the workpiece to be inscribed via two movable mirrors and a flat-field lens.
Predominant use has hitherto been made only of the CO.sub.2 laser (wavelength 10.6 .mu.m) and in particular the Nd:YAG laser (1.06 .mu.m) or frequency-doubled Nd:YAG laser (532 nm). However, the absorption coefficient of the plastics to be treated is frequently not sufficiently high at these wavelengths to induce a color change in the polymeric material. For this reason, suitable auxiliaries and fillers which absorb strongly at the wavelength used and transfer the absorbed energy to the surrounding polymer matrix are usually added to the plastic during preparation thereof (for example addition of sensitizing colorants in the form of pigments, addition of toxic arsenic or cadmium compounds, addition of suitable monomers during copolymerization, coating of the substrate with specific paint and color films, inks, etc.). It is known to include carbon black or antimony trioxide in thermoplastic elastomers. Specific plastics additives having a high absorption capacity, in particular for the wavelength of the Nd:YAG laser, enable marks of high contrast, good contour sharpness and good abrasion resistance (C. Herkt-Maetzky, Kunststoffe 81 (1991) 4). Other processes use radiation-sensitive, bleachable additives (and possibly additional, less radiation-sensitive compounds which cannot be discolored). Irradiation then destroys the radiation-sensitive dyes, leaving the background or complementary color of the polymer matrix in the irradiated areas, forming a visual, colored contrast mark. Such color changes result in good contrast (EP 327508). Plastics which contain commercially available colored pigments can in some cases be inscribed using the frequency-doubled Nd:YAG laser, since many pigments and dyes absorb at 532 nm. The pigments are bleached, resulting in a color change. Addition of modified mica pigments allows the preparation of molding compositions which can be inscribed using the CO.sub.2 laser (C. Herkt-Maetzky, Kunststoffe 81 (1991) 4). A process has also been described in which pigments are applied to a support material and are thermally melted into the surface using the laser (Plastverarbeiter 38 (1987) 64).
EP-A-400 305 describes highly polymeric materials containing copper(II) hydroxide phosphate as additive which can be inscribed using laser light.
Doping of plastics is complicated. The fillers added can have an adverse effect on the material properties of the plastic. For example, additives for improving the inscribability can frequently result in unintended side effects, such as reduced strength of the material or undesired colors on use of pigments. For this reason, specific laser-sensitive additives are desired which, when incorporated into the plastic, do not affect other applicational properties of the material. However, the sensitization is frequently only for a wavelength to which the system has been optimized.
It is an object of the present invention to provide a process for the inscription of moldings based on thermoplastics which does not have the above-described disadvantages and which enables the production of durable, high-contrast inscriptions in a technically simple manner.